Apparatus and method for encoding or decoding an audio signal using a transient-location dependent overlap

ABSTRACT

An apparatus for encoding an audio or image signal, includes: a controllable windower for windowing the audio or image signal to provide the sequence of blocks of windowed samples; a converter for converting the sequence of blocks of windowed samples into a spectral representation including a sequence of frames of spectral values; a transient location detector for identifying a location of a transient within a transient look-ahead region of a frame; and a controller for controlling the controllable windower to apply a specific window having a specified overlap length to the audio or image signal in response to an identified location of the transient, wherein the controller is configured to select the specific window from a group of at least three windows, wherein the specific window is selected based on the transient location.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 15/660,902, filed Jul. 26, 2017, which in turn is acontinuation of copending U.S. application Ser. No. 14/830,484, filedAug. 19, 2015, which in turn is a continuation of copendingInternational Application No. PCT/EP2014/053293, filed Feb. 20, 2014,which is incorporated herein by reference in its entirety, andadditionally claims priority from U.S. Application No. 61/767,115, filedFeb. 20, 2013, which is also incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates to the processing of audio or imagesignals and, in particular, to the encoding or decoding of audio orimage signals in the presence of transients.

Contemporary frequency-domain speech/audio coding schemes based onoverlapping FFTs or the modified discrete cosine transform (MDCT) offersome degree of adaptation to non-stationary signal characteristics. Thegeneral-purpose codecs standardized in MPEG, namely MPEG-1 Layer 3better known as MP3, MPEG-4 (HE-)AAC [1], and most recently, MPEG-0xHE-AAC (USAC), as well as the Opus/Celt codec specified by the IETF[2], allow the coding of a frame using one of at least two differenttransform lengths—one long transform of length M for stationary signalpassages, or 8 short transforms of length M/8 each. In the case of theMPEG codecs, switching from long to short and from short to longtransforms (also known as block switching) necessitates the use ofasymmetrically windowed transition transforms, namely a start and a stopwindow, respectively. These transform shapes, along with other knownprior-art shapes, are depicted in FIG. 16. It should be noted that thelinear overlap slope is merely illustrative and varies in exact shape.Possible window shapes are given in the AAC standard [1] and in section6 of [3].

Given that if the upcoming frame is to be coded with short transforms byan MPEG encoder, the current frame has to be coded with a starttransition transform, it becomes evident that an encoder implementedaccording to one of the above-mentioned MPEG standards necessitates atleast one frame length of look-ahead. In low-delay communicationapplications, however, it is desirable to minimize or even avoid thisadditional look-ahead. To this end, two modifications to thegeneral-purpose coding paradigm have been proposed. One, which wasadopted e.g. in Celt [2], is to reduce the overlap of the long transformto that of the short transform so that asymmetric transition windows canbe avoided. The other modification, which is used e.g. in the MPEG-4(Enhanced) Low Delay AAC coding schemes, is to disallow switching toshorter transforms and instead rely on a Temporal Noise Shaping (TNS)coding tool [4] operating on the long-transform coefficients to minimizetemporal spread of coding error around transients.

Furthermore, like xHE-AAC, Low Delay AAC allows the use of two frameoverlap widths—the default 50% overlap for stationary input, or areduced overlap (similar to the short overlap of the transitiontransforms) for non-stationary signals. The reduced overlap effectivelylimits the time extension of a transform and, thus, its coding error incase of coefficient quantization.

U.S. patent 2008/0140428A 1 assigned to Samsung Electronics Co., as wellas U.S. Pat. Nos. 5,502,789 and 5,819,214 assigned to Sony Corp.,disclose signal-adaptive window or transform size determining units.However, the transformer units controlled by said window or transformsize determining units operate on QMF or LOT sub-band values (implyingthat the described systems both employ cascaded filter-banks ortransforms) as opposed to working directly on the time-domain full-bandinput signal as in the present case. Moreover, in 2008/0140428A 1 nodetails about the shape or control of the window overlap are described,and in U.S. Pat. No. 5,819,214 the overlap shapes follow—i.e., are theresult of—output from the transform size determining unit, which is theopposite of what an embodiment of the current invention proposes.

U.S. patent 2010/0076754A1 assigned to France Telecom follows the samemotivation as the present invention, namely being able to performtransform length switching in communication coding scenarios to improvecoding of transient signal segments, and doing so without extra encoderlook-ahead. However, whereas said document reveals that the low-delayobjective is achieved by avoiding transform-length transition windowsand by post-processing the reconstructed signal in the decoder(disadvantageously by amplification of parts of the decoded signal andthus the coding error), the present invention proposes a simplemodification of the transition window of a conventional system to beintroduced below, such that additional encoder look-ahead can beminimized and special (risky) decoder post-processing can be avoided.

The transition transform to which an inventive modification is to beapplied is the start window described in two variants in U.S. Pat. No.5,848,391 assigned to Fraunhofer-Gesellschaft e.V. and DolbyLaboratories Licensing Corp. as well as, in a slightly different form,in U.S. patent 2006/0122825A 1 assigned to Samsung Electronics Co. FIG.16 shows these start windows and reveals that the difference betweenFraunhofer/Dolby's windows and Samsung's window is the presence of anon-overlapping segment, i.e. a region of the window having a constantmaximum value which does not belong to any overlap slope. TheFraunhofer/Dolby windows exhibit such a “non-overlapping part having alength”, the Samsung windows do not. It can be concluded that an encoderwith the least amount of additional look-ahead but using conventionaltransform switching can be realized by employing Samsung's transitionwindow approach. With such transforms, a look-ahead equal to the overlapwidth between the short transforms suffices to fully switch from long toshort transforms early enough before a signal transient.

Further conventional technology can be found in WO 90/09063 or “Codingof audio signals with overlap block transform and adaptive windowfunctions”, Frequenz, Band 43, September 1989, pages 2052 to 2056 or inAES Convention Paper 4929, “MPEG-4 Low Delay Audio Coding based on theAAC Codec”, E. Allamanche, et al., 106 Convention, 1999.

Nonetheless, depending on the length of the short transform thelook-ahead can remain fairly large and should not be avoided. FIG. 17illustrates the block switching performance during the worst-case inputsituation, namely the presence of a sudden transient at the start of thelook-ahead region, which in turn begins at the end of the long slope,i.e. the overlap region between the frames. According to the prior-artapproaches, at least one of the two depicted transients reaches into thetransition transform. In a lossy coding system utilizing an encoderwithout additional look-ahead—an encoder which does not “see thetransient coming”—this condition causes temporal spreading of the codingerror up to the beginning of the long slope and, even when using TNS,pre-echo noise is thus likely to be audible in the decoded signal.

The two previously mentioned look-ahead work-arounds have theirdisadvantages. Reducing the long-transform overlap by a factor of up to8 on the one hand, as done in the Celt coder, severely limits theefficiency (i.e. coding gain, spectral compaction) on stationary,especially highly tonal, input material. Prohibiting short transforms asin (Enhanced) Low Delay AAC, on the other hand, reduces codecperformance on strong transients with durations of much less than theframe length, often leading to audible pre- or post-echo noise even whenusing TNS.

Thus, the conventional window sequence determination procedures aresub-optimum with respect to flexibility due to the restricted windowlengths, are sub-optimum with respect to the necessitated delay due tothe minimum necessitated transient look-ahead periods, are sub-optimumwith respect to audio quality due to pre- and post-echoes, aresub-optimum with respect to efficiency due to potentially necessitatedadditional pre-processing using additional functionalities apart fromwindowing procedures with certain windows or are sub-optimum withrespect to flexibility and efficiency due to the potential necessity ofchanging a frame/block raster in the presence of a transient.

SUMMARY

According to an embodiment, an apparatus for encoding an audio or imagesignal may have: a controllable windower for windowing the audio orimage signal to provide a sequence of blocks of windowed samples; aconverter for converting the sequence of blocks of windowed samples intoa spectral representation including a sequence of frames of spectralvalues; a transient location detector for identifying a location of atransient within a transient look-ahead region of a frame; and acontroller for controlling the controllable windower to apply a specificwindow having a specified overlap length to the audio or image signal inresponse to an identified location of the transient, wherein thecontroller is configured to select the specific window from a group ofat least three windows including a first window having a first overlaplength, a second window having a second overlap length, and a thirdwindow having a third overlap length or having no overlap, wherein thefirst overlap length is greater than the second overlap length, andwherein the second overlap length is greater than the third overlaplength or greater than an overlap of zero, wherein the specific windowis selected based on the transient location such that one of twotime-adjacent overlapping windows has first window coefficients at thelocation of the transient and the other of the two time-adjacentoverlapping windows has second window coefficients at the location ofthe transient, wherein the second coefficients are at least nine timesgreater than the first coefficients.

According to another embodiment, a decoder for decoding an audio orimage signal including a sequence of blocks of converted windowedsamples and associated window information identifying a specific windowfunction for a block out of at least three different window functionsmay have: a processor for providing a sequence of blocks of spectralvalues; a controllable converter for converting the sequence of blocksof spectral values into a time domain representation using anoverlap-add processing, wherein the controllable converter is controlledby the window information to apply window functions indicated by thewindow information to the corresponding block to calculate a decodedaudio or image signal, wherein the window is selected from a group of atleast three windows including a first window having a first overlaplength, a second window having a second overlap length, and a thirdwindow having a third overlap length or having no overlap, wherein thefirst overlap length is greater than the second overlap length, andwherein the second overlap length is greater than the third overlaplength or greater than an overlap of zero.

According to another embodiment, a method for encoding an audio or imagesignal may have the steps of: windowing the audio or image signal toprovide a sequence of blocks of windowed samples; converting thesequence of blocks of windowed samples into a spectral representationincluding a sequence of frames of spectral values; identifying alocation of a transient within a transient look-ahead region of a frame;and controlling the windowing to apply a specific window having aspecified overlap length to the audio or image signal in response to anidentified location of the transient, wherein the specific window isselected from a group of at least three windows including a first windowhaving a first overlap length, a second window having a second overlaplength, and a third window having a third overlap length or having nooverlap, wherein the first overlap length is greater than the secondoverlap length, and wherein the second overlap length is greater thanthe third overlap length or greater than an overlap of zero, wherein thespecific window is selected based on the transient location such thatone of two time-adjacent overlapping windows has first windowcoefficients at the location of the transient and the other of the twotime-adjacent overlapping windows has second window coefficients at thelocation of the transient, wherein the second coefficients are at leastnine times greater than the first coefficients.

According to another embodiment, a method for decoding an audio or imagesignal including a sequence of blocks of converted windowed samples andassociated window information identifying a specific window function fora block out of at least three different window functions may have thesteps of: providing a sequence of blocks of spectral values; convertingthe sequence of blocks of spectral values into a time domainrepresentation using an overlap-add processing, wherein the convertingis controlled by the window information to apply window functionsindicated by the window information to the corresponding block tocalculate a decoded audio or image signal, wherein the window isselected from a group of at least three windows including a first windowhaving a first overlap length, a second window having a second overlaplength, and a third window having a third overlap length or having nooverlap, wherein the first overlap length is greater than the secondoverlap length, and wherein the second overlap length is greater thanthe third overlap length or greater than an overlap of zero.

Another embodiment may have a computer program for performing, whenrunning on a computer or a processor, the inventive methods.

Aspects of the present invention rely on the finding that, in order fora low-delay audio or image codec to be able to approach the codingquality of general-purpose codecs, it is useful to maintain a highoverlap percentage between long transforms during stationary signalinputs and to allow instant switching to shorter overlaps and transformsat audio or image signal portions surrounding signal non-stationarities.Furthermore, it is desirable to allow a somewhat greater flexibilitythan offering only a binary choice with respect to overlap width and,additionally or alternatively with respect to transform lengths, suchthat the overlap width or lengths of the transform(s) within a frame canbe accurately adapted based on the location of a possible transientwithin the temporal region of the frame in order to minimize pre-echoesor other artifacts.

Specifically, a transient location detector is configured foridentifying a location of a transient within a transient-look-aheadregion of a frame and, based on the location of the transient within theframe, a specific window from a group of at least three windows isselected, where these three windows are different with respect to theiroverlap lengths with corresponding adjacent windows. Thus, the firstwindow has an overlap length being greater than the second window andthe second window has an overlap length being greater than the overlaplength of the third window and the third window can, alternatively, alsohave a zero overlap, i.e., no overlap. The specific window is selectedbased on the transient location such that one of two time-adjacentoverlapping windows has first widow coefficients at the location of thetransient and the other one of the two time-adjacent overlapping windowshas second window coefficients at the location of the transient, whereinthe second coefficients are at least nine times greater than the firstcoefficients. Thus, it is made sure that the transient location is, withrespect to the first window, sufficiently suppressed and the transientis, with respect to the second window, sufficiently captured. In otherwords, and advantageously, the earlier window is already at values closeto zero in the transient location where the transient has been detectedand the second window has window coefficients close or equal to one inthis region so that, during at least a portion of the transient, thetransient is suppressed in the earlier window and is not suppressed inthe later or following window.

In an implementation, the overlap lengths are different by integerfactors so that the second overlap length is, for example, equal to onehalf of the third overlap length and the third overlap length is equalto one half of the second overlap length or is different from the secondoverlap length by a different factor but is greater than or equal to atleast 64 samples or is greater than or equal to at least 32 samples oris greater than or equal to at least even 16 audio or image samples.

The window selection derived from the transient location is transmittedtogether with the frames of the audio or image signal so that a decodercan select the corresponding synthesis windows in line with the encoderselection of the analysis windows, making sure that encoder and decoderare synchronized throughout the whole encoding/decoding operation.

In an implementation, a controllable windower, a converter, a transientlocation detector and a controller form an apparatus for encoding andthe converter applies any of the known aliasing introducing transformssuch as an MDCT (modified discrete cosine transform), an DST (modifieddiscrete sine transform) or any other similar transform. On thedecoder-side, a processor cooperates with a controllable converter inorder to convert a sequence of blocks of spectral values into a timedomain representation using an overlap-add processing in accordance withwindow sequences indicated by a window information received by thedecoder.

Depending on the implementation, a transform length switching can beimplemented in addition to the transform overlap selection, again basedon the transient location within the frame. By implementing amulti-overlap section in which at least three windows overlap with eachother, a very low delay codec concept is realized which againsubstantially reduces the necessitated transient look-ahead delay withrespect to earlier concepts. In a further implementation, it isadvantageous to firstly perform an overlap selection and to subsequentlyperform a transform length decision in order to determine an overlapcode for each frame. Alternatively, the transform length switchingdecision can be done independent from the overlap width decision and,based on these two decisions, an overlap code is determined. Based onthe overlap code for a current frame and the overlap code of an earlierframe, a window sequence selection for a specific transient is done,based on which an encoder as well as a decoder operate in synchrony witheach other.

In a further aspect, a window sequence controller, a preprocessor and aspectrum converter together constitute an apparatus for generating anencoded signal, where three windows have a multi-overlap portion. Thismulti-overlap portion, in which not only two windows as in theconventional technology but three windows overlap with each other,allows a very low delay concept due to the fact that the necessitateddelay due for the transient look-ahead is further reduced. Acorresponding decoder is formed by a decoder processor, a time converterand a post processor. The post processor and the pre-processor performadditional windowing operations using one and the same auxiliary windowon the encoder side and on the decoder side so that an efficientimplementation can be obtained particularly in mobile devices or lowcost devices in which a necessitated ROM or RAM storage is to be assmall as possible.

Embodiments rely on a specific window sequence and a specificinteraction of windows having different lengths so that a short-lengthwindow is “placed” at the transient in order to avoid long pre- orpost-echoes. For making sure that the multi-overlap portion does notresult in audio or image artifacts, the preprocessor on the encoder sideperforms a windowing operation using the auxiliary window function and apre-processing operation using a folding-in operation to obtain amodified multi-overlap portion which is then transformed into thespectral domain using an aliasing introducing transform. On thedecoder-side, a corresponding post processor is configured forperforming a folding-out operation subsequent to correspondingtransforms into the time representation and, subsequent to thefolding-out operation, a windowing using the auxiliary window functionand a final overlap-adding with a preceding block of samples originatingby a window operation with a long window is performed.

In an embodiment in which a transform overlap selection is performed, anincreased audio or image quality is obtained.

Unlike existing coding systems, which employ only a binary choice oftransform overlap width (large/maximum or small), the embodimentproposes a set of three overlap widths from which an encoder can choosean a per-frame (or optionally, a per-transform) basis: maximum overlap,half overlap, or minimum overlap. The maximum overlap could be equal tothe frame length as for long transforms in AAC, i.e. 50% overlap, butcould also equate to one half of the frame length, i.e. 33% overlap, orless, as will be described in an embodiment. Accordingly, the minimumoverlap could indicate an overlap width of zero, i.e. no overlap, butcould also represent a greater-than-zero overlap of a very small numberof time samples or milliseconds, like said embodiment will demonstrate.Finally, the half overlap could be, but does not necessarily have to be,one half of the maximum overlap.

In particular, according to an aspect of the present invention, anoverlap width determining unit is defined which selects for each frame(or optionally, for each transform within a frame) one of the threepossible overlap widths. More precisely, said overlap width determiningunit has, as an input, the output of a transient detection unit toidentify with sufficient accuracy the position of a transient within thecurrent frame (or optionally, within a transform in the current frame)and to derive an overlap width such that at least one of the twoobjectives is achieved:

-   -   The width is chosen such that only one of the overlapping        transforms contains the transient.    -   Pseudo-transients due to time-aliased TNS shaping of coding        error are strongly suppressed.

In other words, the overlap width is determined with the goal ofpreventing pre- or post-echo distortion around a perceptually codedtransient located in the given frame. It shall be noted that a certaindegree of freedom regarding the means of determining the exact locationof the transient is possible. The time or sub-block index designating atransient location could equal the start (onset) of that transientlocation, as in an embodiment, but it could also be the location of themaximum energy or amplitude, or the center of energy, of the transient.

Furthermore, unlike prior-art coding schemes which derive theinstantaneous inter-transform overlaps from the given selection oftransform lengths for a pair of frames (that is, the overlap widthfollows the output of a transform size determining unit), according toanother aspect of the present invention a coding system can, undercertain conditions to be examined below in an embodiment, control orderive the transform length(s) to be used for a particular frame usingthe overlap width attributed to that frame and, optionally, the overlapwidth of the previous frame (i.e. the transform size follows the data ofthe overlap width determining unit).

In a further embodiment in which a multi-overlap portion is used or atransform length switching is applied, a particularly low-delay conceptis obtained.

An improvement to prior-art block switching schemes is an advantageousmodification to the transition transforms of FIG. 16 which allows theadditional encoder look-ahead necessitated for stable-quality operationduring signal non-stationarities to be reduced by one half. As discussedabove, the start windows proposed by Fraunhofer/Dolby or by Samsung arecharacterized by the presence or absence, respectively, of a“non-overlapping part having a length”. The embodiment goes even furtherand allows the left and right overlap slopes of the transition window toextend into each other. In other words, the modified transitiontransform exhibits a “double-overlapping” region of non-zero length inwhich it overlaps with both the preceding frame's long transform as wellas the following short transform. The resulting shape of the inventivetransition transform is illustrated in FIG. 13. In comparison toSamsung's transition window shown in FIG. 17, it is clear that byallowing a “double-overlap” region in the transform, the short-overlapslope on the right end of the transform can be shifted to the leftby—and thereby the necessitated encoder look-ahead can be reduced by—onehalf of the short-transform overlap width. The reduced length of such amodified transition window bears three crucial advantages whichfacilitate implementation, especially on mobile devices:

The transform kernel, i.e. the length of the coefficient vectorresulting from the lapped time/frequency transform (advantageously theMDCT), is exactly half as long as the width of the overlap regionbetween two long transforms. Given the fact that said long-overlap widthusually equals the frame length or one half of the frame length, thisimplies that the inventive transition window and the subsequent shortwindows fit perfectly into the frame grid and that all transform sizesof the resulting codec are related by an integer power-of-two factor, asseen in FIG. 13.

-   -   Both transient locations depicted in FIG. 17 and again in FIG.        13 lie outside the transition transform, so a temporal smearing        of the coding error due to the transients can be restricted to        within the extension of the first two short windows following        the transform. Hence, contrary to the prior-art Fraunhofer/Dolby        and Samsung schemes, audible pre-echo noise around the        transients is unlikely to occur when using the inventive block        switching approach of FIG. 13.    -   Both encoder and decoder can utilize the exact same windows for        the forward and inverse transforms. In a communication device        performing both encoding and decoding, only one set of window        data thus needs to be stored in ROM. Moreover, special pre- or        post-processing of the signal, which would necessitate        additional program ROM and/or RAM, can also be avoided.

Traditionally, transition windows with a “double-overlap” segment as inthe present invention have not been used in speech or audio or imagecoding, most likely because they were thought of as violating certainprinciples which ensure perfect waveform reconstruction in the absenceof quantization of the transform coefficients. It is, however, possibleto exactly reconstruct the input when using the inventive transitiontransform, and furthermore, no special decoder-side post-processing asin the France Telecom proposal is necessitated.

As a further note, it is worth emphasizing that the usage of saidinventive transition window may be controlled by means of the inventiveoverlap width determining unit instead of, or in addition to, atransform length determining unit.

Subsequently, embodiments of the present invention are discussed andillustrated in more detail. Furthermore, particular reference is made tothe dependent claims in which further embodiments are defined.

Furthermore, the specification specifically illustrates an aspectrelated to transient-location adaptive overlap switching particularlywith respect to FIGS. 1a to 7. A further aspect related to themulti-overlap portion is illustrated and described with respect to FIGS.8a to 15f . These individual aspects can be implemented independent fromeach other, i.e., overlap switching can be applied without amulti-overlap region or the multi-overlap region can be applied withouttransient-location adaptive overlap switching. In an implementation,however, both aspects can be advantageously combined resulting in anencoding/decoding concept having a transient location-adaptive overlapswitching and a multi-overlap region. Such a concept can be additionallyenhanced by a transform length switching procedure, again dependent on atransient location within a transient look-ahead region of a frame. Thetransform length switching can be performed dependent on the overlapwidth determination or independent on the overlap switching.

The present invention is not only useful for audio signals but is alsouseful for video, picture or, generally image signals. For example inthe coding of still images or so called I frames in AVC or less or moreadvances technologies, the present invention can be applied to avoidblocking artefacts. A transient in the image field would be a sharp edgeand a frame would correspond for example to a macroblock. The image isthen two-dimensionally encoded using an aliasing introducing transformand a corresponding spatial overlap. This reduces blocking artefacts onthe one hand and reduces any other artefacts by transient portions,i.e., portions with sharp edges on the other hand. Hence, the subsequentdisclosure equally applies to image signals although not specificallyindicated throughout the disclosure

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1a illustrates an apparatus for encoding in the context of anoverlap switching aspect;

FIG. 1b illustrates an apparatus for decoding for the aspect of theoverlap-switching;

FIG. 1c illustrates a decoder-side implementation of the controllableconverter;

FIG. 1d illustrates a further embodiment of the present inventionimplemented with a mobile device;

FIG. 2a illustrates a window sequence with full overlap between adjacentwindows;

FIG. 2b illustrates a window sequence with half overlap between twoadjacent windows;

FIG. 2c illustrates a window sequence with a quarter overlap betweenadjacent windows and a half overlap between adjacent windows and asubsequent full overlap between adjacent windows;

FIGS. 3a to 3c illustrate different overlap widths for differenttransient locations for an embodiment with a 20 ms transform length suchat TCX 20;

FIGS. 4a to 4g illustrate a selection of transform overlap lengths for a10 ms transform length such as TCX 10 dependent on a transient location;

FIGS. 5a to 5c illustrate an encoding of an overlap width;

FIG. 6a illustrates a coding of the overlap width and the transformlength based on the transient position;

FIG. 6b illustrates a transform length decision table;

FIG. 7 illustrates different window sequences dependent on the previousand current overlap codes;

FIG. 8a illustrates an encoder in the context of a multi-overlap portionin an embodiment of the present invention;

FIG. 8b illustrates a decoder for the aspect of the multi-overlapportion in an embodiment of the present invention;

FIG. 9a illustrates a procedure in accordance with an embodimentillustrating the encoder-side;

FIG. 9b illustrates a flow chart of a procedure performed on theencoder-side;

FIG. 10a illustrates an embodiment of a procedure on the decoder-side;

FIG. 10b illustrates a further embodiment of a procedure performed onthe decoder-side;

FIG. 11a illustrates operations performed on the encoder-side of anembodiment;

FIG. 11b illustrates operations performed by a decoder in an embodimentof the present invention;

FIGS. 12a and 12b illustrate a further embodiment of procedures to beperformed on the encoder/decoder-side in the context of themulti-overlap aspect of the invention;

FIG. 13 illustrates different window sequences both having amulti-overlap portion;

FIG. 14a illustrates a window sequence having a switched transformlength dependent on the transient location;

FIG. 14b illustrates a further window sequence having a multi-overlapportion;

FIGS. 15a to 15f illustrate different window sequences and correspondinglook-ahead portions and pre-echoes;

FIG. 16 illustrates conventional window shapes; and

FIG. 17 illustrates conventional window sequences formed by windowshapes of FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a illustrates an apparatus for encoding an audio signal 100. Theapparatus for encoding an audio signal comprises a controllable windower102 for windowing the audio signal 100 to provide a sequence of blocksof windowed samples at 103. The decoder furthermore comprises aconverter 104 for converting the sequence of blocks of windowed samples103 into a spectral representation comprising a sequence of frames ofspectral values indicated at 105. Furthermore, a transient locationdetector 106 is provided. The detector is configured for identifying alocation of a transient within a transient look-ahead region of a frame.Furthermore, a controller 108 for controlling the controllable windoweris configured for applying a specific window having a specified overlaplength to the audio signal 100 in response to an identified location ofthe transient illustrated at 107. Furthermore, the controller 108 is, inan embodiment, configured to provide window information 112 not only tothe controllable windower 102, but also to an output interface 114 whichprovides, at its output, the encoded audio signal 115. The spectralrepresentation comprising the sequence of frames of spectral values 105is input in an encoding processor 110, which can perform any kind ofencoding operation such as a prediction operation, a temporal noiseshaping operation, a quantizing operation with respect to apsycho-acoustic model or at least with respect to psycho-acousticprinciples or may comprise a redundancy-reducing encoding operation suchas a Huffman encoding operation or an arithmetic encoding operation. Theoutput of the encoding processor 110 is then forwarded to the outputinterface 114 and the output interface 114 then finally provides theencoded audio signal having associated, to each encoded frame, a certainwindow information 112.

The controller 108 is configured to select the specific window from agroup of at least three windows. The group comprises a first windowhaving a first overlap length, a second window having a second overlaplength, and a third window having a third overlap length or no overlap.The first overlap length is greater than the second overlap length andthe second overlap length is greater than a zero overlap. The specificwindow is selected, by the controllable windower 102 based on thetransient location such that one of two time-adjacent overlappingwindows has first window coefficients at the location of the transientand the other of the two time-adjacent overlapping windows has secondwindow coefficients at the location of the transient and the secondwindow coefficients are at least nine times greater than the firstcoefficients. This makes sure that the transient is substantiallysuppressed by the first window having the first (small) coefficients andthe transient is quite unaffected by the second window having the secondwindow coefficients. Advantageously, the first window coefficients areequal to 1 within a tolerance of plus/minus 5%, such as between 0.95 and1.05, and the second window coefficients are advantageously equal to 0or at least smaller than 0.05. The window coefficients can be negativeas well and in this case, the relations and the quantities of the windowcoefficients are related to the absolute magnitude.

FIG. 2a illustrates a window sequence with first windows only and thefirst windows have the first overlap length. Particularly, the lastframe has associated a first window 200, the current frame hasassociated window 202 and the third or next frame has associated awindow 204. In this embodiment, adjacent windows overlap by 50%, i.e., afull length. Furthermore, the frames are placed with respect to thewindows in order to identify which portion of the audio signal isprocessed by a frame. This is explained referring to the current frame.The current frame has a left portion 205 a and a right portion 205 b.Correspondingly, the last frame has a right portion 204 b and a leftportion 204 a. Analogously, the next frame has a left portion 206 a anda right portion 206 b. Left/right refers to earlier in time and later intime as illustrated in FIG. 2a . When the current frame of spectralvalues is generated, the audio samples obtained by windowing with thewindow 202 are used. The audio samples stem from portions 204 b to 206a.

As known in the art of MDCT processing, generally, processing using analiasing-introducing transform, this aliasing-introducing transform canbe separated into a folding-in step and a subsequent transform stepusing a certain non-aliasing introducing transform. In the FIG. 2aexample, section 204 b is folded into section 205 a and section 206 a isfolded into section 205 b. The result of the folding operation, i.e.,the weighted combination of 205 a, 204 b on the one hand and 206 a and205 b are then transformed into the spectral domain using a transformsuch as a DCT transform. In the case of an MDCT, a DCT IV transform isapplied.

Subsequently, this is exemplified by reference to the MDCT, but otheraliasing-introducing transforms can be processed in a similar andanalogous manner. As a lapped transform, the MDCT is a bit unusualcompared to other Fourier-related transforms in that it has half as manyoutputs as inputs (instead of the same number). In particular, it is alinear function F: R^(2N)→R^(N) (where R denotes the set of realnumbers). The 2N real numbers x0, . . . , x2N−1 are transformed into theN real numbers X0, . . . , XN−1 according to the formula:

$X_{k} = {\sum\limits_{n = 0}^{{2N} - 1}{x_{n}{\cos \left\lbrack {\frac{\pi}{N}\left( {n + \frac{1}{2} + \frac{N}{2}} \right)\left( {k + \frac{1}{2}} \right)} \right\rbrack}}}$

(The normalization coefficient in front of this transform, here unity,is an arbitrary convention and differs between treatments. Only theproduct of the normalizations of the MDCT and the IMDCT, below, isconstrained.)

Inverse Transform

The inverse MDCT is known as the IMDCT. Because there are differentnumbers of inputs and outputs, at first glance it might seem that theMDCT should not be invertible. However, perfect invertibility isachieved by adding the overlapped IMDCTs of time-adjacent overlappingblocks, causing the errors to cancel and the original data to beretrieved; this technique is known as time-domain aliasing cancellation(TDAC).

The IMDCT transforms N real numbers X0, . . . , XN−1 into 2N realnumbers y0, . . . , y2N−1 according to the formula:

$y_{n} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{X_{k}{\cos \left\lbrack {\frac{\pi}{N}\left( {n + \frac{1}{2} + \frac{N}{2}} \right)\left( {k + \frac{1}{2}} \right)} \right\rbrack}}}}$

(Like for the DCT-IV, an orthogonal transform, the inverse has the sameform as the forward transform.)

In the case of a windowed MDCT with the usual window normalization (seebelow), the normalization coefficient in front of the IMDCT should bemultiplied by 2 (i.e., becoming 2/N).

In typical signal-compression applications, the transform properties arefurther improved by using a window function wn (n=0, . . . , 2N−1) thatis multiplied with xn and yn in the MDCT and IMDCT formulas, above, inorder to avoid discontinuities at the n=0 and 2N boundaries by makingthe function go smoothly to zero at those points. (That is, we windowthe data before the MDCT and after the IMDCT.) In principle, x and ycould have different window functions, and the window function couldalso change from one block to the next (especially for the case wheredata blocks of different sizes are combined), but for simplicity weconsider the common case of identical window functions for equal-sizedblocks.

The transform remains invertible (that is, TDAC works), for a symmetricwindow wn=w2N−1−n, as long as w satisfies the Princen-Bradley condition:

w _(n) ² +w _(n+N) ²=1

various window functions are used. A window that produces a form knownas a modulated lapped transform [3][4] is given by

$w_{n} = {\sin \left\lbrack {\frac{\pi}{2N}\left( {n + \frac{1}{2}} \right)} \right\rbrack}$

and is used for MP3 and MPEG-2 AAC, and

$w_{n} = {\sin \left( {\frac{\pi}{2}{\sin^{2}\left\lbrack {\frac{\pi}{2N}\left( {n + \frac{1}{2}} \right)} \right\rbrack}} \right)}$

for Vorbis. AC-3 uses a Kaiser-Bessel derived (KBD) window, and MPEG-4AAC can also use a KBD window.

Note that windows applied to the MDCT are different from windows usedfor some other types of signal analysis, since they have to fulfill thePrincen-Bradley condition. One of the reasons for this difference isthat MDCT windows are applied twice, for both the MDCT (analysis) andthe IMDCT (synthesis).

As can be seen by inspection of the definitions, for even N the MDCT isessentially equivalent to a DCT-IV, where the input is shifted by N/2and two N-blocks of data are transformed at once. By examining thisequivalence more carefully, important properties like TDAC can be easilyderived.

In order to define the precise relationship to the DCT-IV, one has torealize that the DCT-IV corresponds to alternating even/odd boundaryconditions: even at its left boundary (around n=−½), odd at its rightboundary (around n=N−½), and so on (instead of periodic boundaries asfor a DFT). This follows from the identities and. Thus, if its inputs

${\cos \left\lbrack {\frac{\pi}{N}\left( {{- n} - 1 + \frac{1}{2}} \right)\left( {k + \frac{1}{2}} \right)} \right\rbrack} = {{\cos \left\lbrack {\frac{\pi}{N}\left( {n + \frac{1}{2}} \right)\left( {k + \frac{1}{2}} \right)} \right\rbrack}\mspace{14mu} {and}}$${\cos \left\lbrack {\frac{\pi}{N}\left( {{2N} - n - 1 + \frac{1}{2}} \right)\left( {k + \frac{1}{2}} \right)} \right\rbrack} = {- {{\cos \left\lbrack {\frac{\pi}{N}\left( {n + \frac{1}{2}} \right)\left( {k + \frac{1}{2}} \right)} \right\rbrack}.}}$

Thus, if its inputs are an array x of length N, we can imagine extendingthis array to (x, −xR, −x, xR, . . . ) and so on, where xR denotes x inreverse order.

Consider an MDCT with 2N inputs and N outputs, where we divide theinputs into four blocks (a, b, c, d) each of size N/2. If we shift theseto the right by N/2 (from the +N/2 term in the MDCT definition), then(b, c, d) extend past the end of the N DCT-IV inputs, so we have to“fold” them back according to the boundary conditions described above.

Thus, the MDCT of 2N inputs (a, b, c, d) is exactly equivalent to aDCT-IV of the N inputs: (−cR−d, a−bR), where R denotes reversal asabove.

This is exemplified for window function 202 in FIG. 2a . a is theportion 204 b, b is the portion 205 a, c is the portion 205 b and d isthe portion 206 a.

(In this way, any algorithm to compute the DCT-IV can be triviallyapplied to the MDCT.) Similarly, the IMDCT formula above is precisely ½of the DCT-IV (which is its own inverse), where the output is extended(via the boundary conditions) to a length 2N and shifted back to theleft by N/2. The inverse DCT-IV would simply give back the inputs(−cR−d, a−bR) from above. When this is extended via the boundaryconditions and shifted, one obtains:

IMDCT(MDCT(a,b,c,d))=(a−bR,b−aR,c+dR,d+cR)/2.

Half of the IMDCT outputs are thus redundant, as b−aR=−(a−bR)R, andlikewise for the last two terms. If we group the input into biggerblocks A,B of size N, where A=(a, b) and B=(c, d), we can write thisresult in a simpler way:

IMDCT(MDCT(A,B))=(A−AR,B+BR)/2

One can now understand how TDAC works. Suppose that one computes theMDCT of the time-adjacent, 50% overlapped, 2N block (B, C). The IMDCTwill then yield, analogous to the above: (B−BR, C+CR)/2. When this isadded with the previous IMDCT result in the overlapping half, thereversed terms cancel and one obtains simply B, recovering the originaldata.

The origin of the term “time-domain aliasing cancellation” is now clear.The use of input data that extend beyond the boundaries of the logicalDCT-IV causes the data to be aliased in the same way that frequenciesbeyond the Nyquist frequency are aliased to lower frequencies, exceptthat this aliasing occurs in the time domain instead of the frequencydomain: we cannot distinguish the contributions of a and of bR to theMDCT of (a, b, c, d), or equivalently, to the result of IMDCT(MDCT(a, b,c, d))=(a−bR, b−aR, c+dR, d+cR)/2. The combinations c−dR and so on, haveprecisely the right signs for the combinations to cancel when they areadded.

For odd N (which are rarely used in practice), N/2 is not an integer sothe MDCT is not simply a shift permutation of a DCT-IV. In this case,the additional shift by half a sample means that the MDCT/IMDCT becomesequivalent to the DCT-III/II, and the analysis is analogous to theabove.

We have seen above that the MDCT of 2N inputs (a, b, c, d) is equivalentto a DCT-IV of the N inputs (−cR−d, a−bR). The DCT-IV is designed forthe case where the function at the right boundary is odd, and thereforethe values near the right boundary are close to 0. If the input signalis smooth, this is the case: the rightmost components of a and bR areconsecutive in the input sequence (a, b, c, d), and therefore theirdifference is small. Let us look at the middle of the interval: if werewrite the above expression as (−cR−d, a−bR)=(−d, a)−(b,c)R, the secondterm, (b,c)R, gives a smooth transition in the middle. However, in thefirst term, (−d, a), there is a potential discontinuity where the rightend of −d meets the left end of a. This is the reason for using a windowfunction that reduces the components near the boundaries of the inputsequence (a, b, c, d) towards 0.

Above, the TDAC property was proved for the ordinary MDCT, showing thatadding IMDCTs of time-adjacent blocks in their overlapping half recoversthe original data. The derivation of this inverse property for thewindowed MDCT is only slightly more complicated.

Consider to overlapping consecutive sets of 2N inputs (A,B) and (B,C),for blocks A,B,C of size N. Recall from above that when (A, B) and (B,C)are MDCTed, IMDCTed, and added in their overlapping half, we obtain(B+B_(R))/2+(B−B_(R))/2=B, the original data. Now we suppose that wemultiply both the MDCT inputs and the IMDCT outputs by a window functionof length 2N. As above, we assume a symmetric window function, which istherefore of the form (W,W_(R)) where W is a length-N vector and Rdenotes reversal as before. Then the Princen-Bradley condition can bewritten as W+W_(R) ²=(1, 1, . . . ), with the squares and additionsperformed elementwise.

Therefore, instead of MDCTing (A,B), one now MDCTs (WA, W_(R)B) with allmultiplications performed elementwise. When this is IMDCTed andmultiplied again (elementwise) by the window function, the last-N halfbecomes:

W _(R)·(W _(R) B+(W _(R) B)_(R))=W _(R)·(W _(R) B+WB _(R))=W _(R) ² B+WW_(R) B _(R).

(Note that we no longer have the multiplication by ½, because the IMDCTnormalization differs by a factor of 2 in the windowed case.)

Similarly, the windowed MDCT and IMDCT of (B,C)

yields, in its first-N half:

W·(WB−W _(R) B _(R))=W ² B−WW _(R) B _(R)

When one adds these two halves together, one recovers the original data.

In a similar procedure, the next frame is calculated by using portions205 b, 206 a, 206 b and the first portion of the next to next frame inFIG. 2a . Thus, windows 200, 202, 204 correspond to the window functionhaving a first overlap length of the three windows with the differentoverlap lengths used by the controllable windower 102 of FIG. 1a . Asstated, FIG. 2a illustrates a situation, where no transients aredetected in the last frame, the current frame and the next frame and,specifically, in the look-ahead region for each frame indicated by item207 for the last frame, 208 for the current frame and 209 for the nextframe. FIG. 2b illustrates a situation, where transients are detected attransient positions 210, 211, 212, 213. Due to the fact that a transientposition is, for example, detected at 210, and due to the fact that 210is in the look-ahead region starting at 207 for the last frame, thecontroller 108 determines that a switch from the first window 201 to afurther window 215 is to be performed. Due to the further transients211, and, particularly, 212/213 which lie in the next look-ahead region,the current frame additionally is processed using the second window 216with the second overlap length. Thus, window 215 is a kind of a startwindow changing from the window with the first overlap length indicatedat 201 over to the second window having the second overlap length. Asillustrated, the second overlap length only extends over eight slotsand, therefore, is only half as long as the first overlap length. Due tothe fact that in the look-ahead region starting at 209, no transient isdetected anymore, a switch is performed back to the long window 201 by akind of a “stop window 217”. Again, it is noted that the overlap lengthillustrated at 218 in the current frame on the one hand and between thecurrent frame and the next frame on the other hand, which is indicatedat 218 is half as long as the overlap length in FIG. 2a for the firstwindow which is 16 illustrated slots.

Thus, the half-overlap window is used for transients which are detectedin detection regions 1 and 6. As illustrated at 219, such a detectionregion comprises two slots. Thus, the look-ahead range is separated intoeight slots. On the other hand, however, a more coarse or more finesubdivision can be performed. However, in embodiments, the look-aheadregion is subdivided into at least four slots and advantageouslysubdivided into eight slots as illustrated in 2 b and 2 c and otherfigures.

As illustrated, the second window 216 has the half overlap at bothsides, while the window 215 has the half overlap on the right side andhas the full overlap on the left side and the window 217 has thehalf-overlap on the left side and the full overlap on the right side.

Reference is made to FIG. 2c . FIG. 2c illustrates a situation, wherethe transient detector detects in the look-ahead region starting in themiddle of the last frame that there is a transient in the secondtransient detection region 222. Thus, a switch to a quarter overlap isperformed in order to make sure that the transient 223 is only “smeared”within the window 224, but is not included in the region defined bywindow 201 or in the region defined by window 225. Furthermore, asequence is indicated, where a switch from a quarter overlap in the lastframe and the current frame to a half overlap between the current frameand the next frame and back to the full overlap between the next frameand the next to next frame is performed. This is due to the detectedtransients. In the look-ahead region starting at 208, transients aredetected in portion one and portion six while transients are detected inportion two and portion five between the last frame 207 and the currentframe 208.

Thus, FIG. 2c illustrates a window sequence, where the first window 201having the full or first overlap length is illustrated, where a secondwindow having the second overlap length indicated at 218 is used, wherethe second window can, for example, be window 225 or window 226, andwhere a third window having a third overlap length is illustrated aswindow 224 or window 225 which has, at its left side, the small overlaplength 229. Thus, a window sequence switching from a full overlap to aquarter overlap and then to a half overlap and then to a full overlap isillustrated. Hence, the first window having the first overlap length canbe an asymmetric window having a different overlap from the firstoverlap on one side and having the first overlap length on the otherside. Alternatively, however, the first window can also be a windowhaving the first overlap length on both sides as illustrated at 216 inFIG. 2b . Furthermore, the second window having the second overlaplength can be a symmetric window having the second overlap length onboth sides or can be an asymmetric window having the second overlaplength on one side and having, at the other side, the first overlaplength or the third overlap length or any other overlap length. Finally,the third window having the third overlap length can be a symmetricwindow having the third overlap length on both sides or can be a windowhaving the third overlap length on one side and having a differentoverlap length on the other side.

Subsequently, further embodiments are illustrated with respect to thefollowing figures. Generally, the detection of the transient and itslocation can be done for example using a method or procedure similar tothe transient detector described in U.S. Pat. No. 6,826,525 B2, but anyother transient detectors can be used as well.

The transient detection unit identifies the presence and, if applicable,the location of the onset of the strongest transient in the new signalportion of a given frame, i.e., excluding the overlap region between thecurrent and the previous frame. The resolution of the index describingthe transient location is, in the following figures, ⅛ of the framelength, so the index range is from 0 to 7. In subsequent figures, thesub-blocks with indices 0, . . . , 7 represent the newest 20 ms of atime domain signal that are used for the coding in the current frame.

FIGS. 3a-3c illustrate the selection of the transform overlap width foran exemplary ms transform length, i.e., for a TCX20 transform length.

In FIG. 3a , a transient is not present in the current frame. Therefore,a full overlap 300 is detected.

FIG. 3b , to the contrary, illustrates a situation, where a transient isdetected in the seventh sub-block so that a half-overlap 302 is selectedby the controller 108 of FIG. 1a . Furthermore, FIG. 3c illustrates thesituation, where a transient is detected in the sixth sub-block and,therefore, a minimum overlap 304 is set by the controller. Thus, thetransient location detector 106 detects whether there is a transient andif not, the overlap width or first overlap width 300 is selected. When,however, there is a transient in the seventh sub-block as determined bythe transient location detector 106 of FIG. 1a , then the second overlaplength 302 being half of the first overlap length 300 is set by thecontroller and when the transient is in the sub-block 6, then a minimumoverlap is set. FIG. 3c additionally shows the situation where, insteadof the fact that the transient is detected in location 6 or 7, thetransform length is nevertheless maintained. Thus, the transform lengthsof windows 301 a, 301 b or 303 a or 303 b are identical and equal to thefirst window having the longest overlap length illustrated in FIG. 3a at301 a and 301 b. As will be shown later on, it is advantageous to notonly control the overlap length, but to additionally control thetransform length specifically in situations, where the transient isdetected in other sub-blocks. Thus, the overlap width between thecurrent and the following transform window depends on the location ofthe transient. The overlap between the current and the previoustransform window, however, was determined when processing the previousframe.

Subsequently, reference is made to FIGS. 4a to 4g in order to show theselection of the transform overlap length for 10 ms transform length,i.e., TCX10. If, for example, a codec is limited to a 10 ms transformlength, the overlap between two TCX10 windows is chosen so that pseudotransients due to time-aliased TNX shaping of the coding error arestrongly suppressed. Also, the smearing of the transient to more thanfive previous and to more than five following sub-blocks is minimized.That is the pre-echo and the post-echo are limited to 12.5 ms. Thechoice of the overlap is based on the transient position.

FIG. 4a illustrates a situation, where a transient is detected in thezeroth or the first sub-block. Then, “first windows” 401, 402 arechosen, which have the maximum or first overlap length 403. Furthermore,for illustrative purposes, a TCX20 full overlap with the previous andwith the next window is illustrated as a reference at 404. Thus, the“full overlap” corresponds to 50% of the window 401, 402 or corresponds33% of the TCX20 window 301 a, 301 b, for example. Thus, the overlaplength 300 in FIGS. 3a and 403 in FIG. 4a are identical.

FIG. 4b illustrates a situation that a transient is detected in thesecond sub-block and the controller then controls the window sequence sothat a minimum overlap 404 corresponding to the “third overlap length”illustrated at 229 of FIG. 2c is chosen. Thus, windows 406, 407 whichare, in this embodiment, asymmetric windows are selected having theshort overlap length corresponding to the “second window” in thelanguage of FIGS. 1a and 1b . Furthermore, when the transient in thethird sub-block is detected, then the second overlap length 405 isselected. Thus, windows 408, 409 correspond to the third window havingthe third overlap length 405, but are asymmetric windows.

Furthermore, as illustrated in FIG. 4d , the full overlap length isdetermined when the transient is at transient portion 4 and, therefore,the windows selected in this situation are the windows 401, 402illustrated in FIG. 4a . When choosing the overlap so that on the one ofthe overlapping transforms contains a transient as illustrated, the casewherein the transient is in the second or third sub-block is asillustrated in FIG. 4f or 4 g respectively. The cases, when thetransient is in the zero or first sub-block are then treated separately,as well as the cases, when the transient is in the fourth or fifthsub-block. Therefore, reference is made to FIG. 4e illustrating thesituation, where the transient is in the zero sub-block, a windowsequence as illustrated in FIG. 4e is obtained, where there is ahalf-overlap 405 and which is then switched back to the full overlap403. This is obtained by the window sequence formed by the start window408, and the stop window 409 and a further normal length window 402.

FIG. 4f , on the other hand, illustrates the situation, where thetransient is in the first sub-block so that a short or a third overlaplength 404 is selected, which is made possible by the start window 406and the stop window 407 which is then followed by a full overlap window402. Thus, window 408 or 409 in FIG. 4e illustrates the second windowhaving the second overlap length 405 and window 406 and 407 correspondto the third window having the third overlap length 404″.

FIG. 4g illustrates a situation, where the transient is detected to liein the fourth sub-block. This situation is reflected by a first window401 having a full overlap length 403 and a second window 409 having ahalf-overlap length 405 and a further second window 414 having thesecond overlap length 405. The right side of the window 414, however,depends on the overlap length determined for the next frame, i.e., inthe next look-ahead region starting at the time instant indicated byreference number 415.

Thus, FIGS. 4a-4g illustrate the situation, where the overlap length isdetermined so that the transient is located only within one window whichis made sure by the fact that, at the location of the transient, forexample in sub-block 4, the window coefficients of window 414 are equalto 0 and the window coefficients of window 409 are equal to 1.

Subsequently, reference is made to an embodiment, in which the transformlength is derived from the overlap width. FIGS. 5a, 5b, 5c illustratethree different overlap lengths 403, 405, 404, where the full overlaplength is determined by two first windows indicated at 501 and 502.Furthermore, the half overlap length is obtained by two second windowshaving the second overlap length illustrated at 503 and 504, and thethird overlap length 404 is obtained by two third windows 505 and 506having the third overlap length 404. The full overlap is codedadvantageously using a “0” bit, the half overlap is coded using a “11”bit combination and the minimal overlap is coded using the “10” bitcombination.

Thus, this coding is useful when determining the overlap width andtransform length selection when TCX-20 and a combination of TCX-5 andTCX-10 frames can be used.

Unlike coding schemes which derive the instantaneous inter-transformoverlaps from the given selection of transform lengths for a pair offrames, that is the overlap width follows the output of the transformlength determination, an embodiment of the present invention relates toa coding system which can control or derive the transform length(s) tobe used for a particular frame using the overlap width attributed tothat frame and optionally the overlap width of a previous frame, i.e.the transform length follows the data of the overlap width determiningunit or, with respect to FIG. 1a , by the cooperation of the transientlocation detector 106 and the controller 108. FIG. 6a illustrates acoding table and FIG. 6b illustrates a corresponding decision table. InFIGS. 5a, 5b and 5c , the full line represents the right half of thewindow of the last transform in the current frame and the broken linerepresents the left half of the window of the first transform in thefollowing frame.

FIG. 6a illustrates a coding of the overlap and the transform lengthbased on the transient position. In particular, the short/long transformdecision is coded using 1 bit as indicated in column 600 and the overlapwith the first window of the following frame is coded using the variablelength code with 1 or 2 bits as illustrated at column 602. The code forthe short/long transform decision 600 on the one hand and the binarycode for the overlap width of column 602 are concatenated to obtain theso called overlap code in column 603. Furthermore, the overlap with thefirst window of the following frame is determined by the controller 108depending on the transient position index of column 604 as determined bythe transient detector 106. In contrast to the earlier illustrations,the transient position index has an increased look-ahead range startingat two earlier slots indicated by −1 and −2 and for this situation,additionally, the full overlap is signaled in this embodiment.

Thus, the full overlap is signaled for “no transient” or a transientposition between −2 and 1. Furthermore, a half overlap is signaled bycolumn 605 for transient positions 2 and 3 and 7 and the minimal overlapis signaled for transient positions 4, 5, 6.

Thus, the index “−2” in FIG. 6a means that there was a transient in theprevious frame at position 6, and “−1” means that there was a transientin the previous frame at position 7. As stated, “none” means that notransient was detected in the transient look-ahead region.

As outlined, the short/long transform decision and the overlap width arejointly coded using the overlap code. The overlap code consists of 1 bitfor a short/long transform decision and of the binary code for theoverlap width coded with 1 or 2 bits. The code is a variable length codewhere it is automatically detected where a codeword starts and theearlier codeword stops. The codes for the short/long transform decisionand for the overlap width are defined in FIG. 6a . For example, when theshort/long transform decision gives 1 and the minimal overlap isselected, i.e. a binary code is equal to 10, the overlap code is 110.

Furthermore, FIG. 6a illustrates the situation that a short transformdecision is taken for all transient positions between −2 and 5 and along transform is chosen for no transient or the transient at position 6or 7. Thus, FIG. 6a illustrates the situation where the transientlocation detector can detect a certain transient at a certain position,and where independent form each other or in parallel, the short/longtransform decision and the overlap with the first window of thefollowing frame can be determined, i.e. the full overlap code 603 can bederived. It is emphasized that those skilled in the art will understandthat any other codes for encoding different short/long transforms anddifferent overlaps can be used. Furthermore, more than two, i.e. threeor even more transform lengths can be determined and signaled and, atthe same time, more than three overlaps such as four or five differentoverlap lengths can be determined and encoded as well. All thisdetermined is, for example, in response to a transient location detectoroperating on at least four different divisions per frame or, as in theembodiment, operating on eight divisions per frame or, for a finerdecision, operating in even more divisions, such as sixteen divisions ofa frame.

Based on the overlap code for the current frame and for the previousframe, a decision is made for a combination of the transform length touse, as illustrated in FIG. 6b . Thus, FIG. 6b illustrates the decisionof a transform length based on the previous overlap code and the currentoverlap code. For example, if the previous overlap code and the currentoverlap code both are “00”, then a window such as 401 is used. If theprevious overlap code was 10 and the current overlap code is 00, thenthe same window is selected. However, if the previous code is 111,meaning a half overlap code, and the current overlap code is 00, thenthe window 409 of FIG. 4c , for example, is selected. For a previousoverlap code of 110 and the current overlap code 00, again a longtransform is selected, but with a window similar to window 407, and thesame situation is for a previous overlap code of 010 and the currentoverlap code of 00, i.e., window 407 of FIG. 4f is selected. Finally,for a previous overlap code 011, and for the current overlap code 00, awindow such as 409 in FIG. 4e is selected.

Other windows are selected for other combinations and this isspecifically illustrated with respect to FIG. 7. Thus, FIG. 7illustrates some of the transform length combinations together with thetransient position in the current frame and with the overlap codes forthe current and for the previous frames. 110/010-111 in FIG. 7 meansthat the previous overlap code is 110 or 010 and the current overlapcode is 111. FIG. 7 therefore illustrates different combinations. Forexample, the upper left picture in FIG. 7 illustrates a minimum overlapat the beginning of a sequence of two TCX-5 transforms and a followingTCX-10 transform having the full overlap. Contrary thereto, the picturebelow this picture illustrates a minimum overlap followed by four TCX-5windows, where the fourth window of the TCX-5 windows has a half overlapand so on. Thus, reference numbers 700, 701 illustrate a sequence of twoTCX-5 or two short windows followed by a medium window. Similarly,reference numbers 702, 703, 704, 705, 706, 707 illustrate a situationwith four short transform lengths or “TCX-5” transforms while referencenumbers 708, 709, 710, 711 illustrate the situation where there is, atthe first time, i.e. at the beginning of the sequence, a mediumtransform length window such as a TXC 10 window followed by two TCX-5 orshort transform length windows. The sequences 700 to 711 in FIG. 7 canbe introduced by other such sequences or by TCX-20 or long transformlength windows having different overlaps such as short overlaps at 700,702 for example, a medium overlap at 704 or long overlaps at 708 or 710,for example. At the same time, the sequence can be followed by furthersuch sequences or can be followed by TCX-20, i.e. long transformwindows, but with a different overlap length.

Thus, the sequence 700, for example, ends with a long overlap and thesequence 702, for example, ends with a medium overlap or the sequence706, for example, ends with a small overlap length.

As illustrated in FIG. 1a , the window information, i.e. the overlapcode 603 of FIG. 6a illustrated at 112 in FIG. 1a may be associated toeach encoded frame by an output interface 114.

Furthermore, the transform applied in the converter 104 may be an MDCTor an MDST or a different aliasing introducing transform which ischaracterized by the fact that the number of spectral values in a blockof spectral values is lower than the number of windowed samples in ablock of windowed samples input into the transform or, with respect tothe decoder side, in which the number of time domain output samples isgreater than the number of spectral values input into such an aliasingreducing back or inverse transform.

As illustrated in all of FIGS. 2 to 7, a constant frame raster ismaintained. Thus, the controller 108 makes sure that even though aswitch over to shorter transform lengths is performed as, for example,illustrated in FIG. 7, the same constant frame raster is maintained.This is made sure by using only such specific windows which result insimilar transform length for each class of windows in the context of thecorrect overlap size. Thus, each TCX-5 transform length is defined tohave such an overlap region and a constant region between the twooverlap regions that the transform results N/4 spectral values, where Nis the number of spectral values within a frame. The form and size andspecifically the overlap lengths of the TCX 20 transform windows areadditionally designed in such a way that this window results in Nspectral samples subsequent to the transform.

FIG. 1c illustrates a decoder-side implementation of the controllableconverter 158. In particular, the controllable converter 158 comprises afrequency-time converter 170, a subsequently connected synthesiswindower 172 and a final overlap-adder 174.

Specifically, the frequency-time converter performs the transform suchas a DCT-IV transform and a subsequent fold-out operation so that theoutput of the frequency-time converter 170 has, for a first or longwindow, 2N samples while the input into the frequency-time converterwas, exemplarily, N spectral values. On the other hand, when the inputinto the frequency-time converter are N/8 spectral values, then theoutput is N/4 time domain values for an MDCT operation, exemplarily.

Then, the output of the frequency-time converter 170 is input into asynthesis windower which applies the synthesis window which isadvantageously exactly the same as the encoder-side window. Thus, eachsample is, before an overlap-add is performed, windowed by two windowsso that the resulting “total windowing” is a square of the correspondingwindow coefficients so that the Princen-Bradley condition as discussedbefore is fulfilled.

Finally, the overlap-adder 174 performs the corresponding correctoverlap-add in order to finally obtain the decoded audio signal atoutput 175. In particular, the frequency-time converter 170, thesynthesis windower 172 and the overlap-adder 174 are controllable andare controlled, for example, by the overlap code 603 discussed in thecontext of FIG. 6a or by any other information referring to thesituation discussed in the context of FIG. 6b . However, thecorresponding transform length for the frequency-time converter isdetermined, based on the previous overlap code and the current overlapcode using the transform length decision table. Furthermore, the windowsize/shape is also determined based on the previous overlap code and acurrent overlap code and the same is true for the overlap-adder so thatthe overlap-adder applies the maximum overlap, the medium overlap or theminimum overlap as signalled.

Thus, it is advantageous that controller 180 in the decoder in FIG. 1creceives the overlap codes, i.e., the previous overlap code 606 and thecurrent overlap code 607 and determines, from this information, theoverlap and window for the block of spectral values.

Thus, each window and the corresponding transform size associated withthe window are determined. In the embodiments where an MDCT is used as atransform and an inverse MDCT is used for the inverse transform, thewindow size is two times the transform length or the transform length ishalf of the window size.

FIG. 1d illustrates a further embodiment of the present inventionimplemented with a mobile device, where the mobile device comprises, onthe one hand, an encoder 195 and on the other hand a decoder 196.Furthermore, in accordance with an embodiment of the present invention,both the encoder 105 and the decoder 106 retrieve the same windowinformation from only a single memory 197, since the windows used in theencoder 195 and the windows used in the decoder 196 are identical toeach other. Thus, the decoder has a read-only memory 197 or a randomaccess memory or generally any memory 197 in which only a single set ofwindow sequences or windows is stored for usage both in the encoder andin the decoder. This is advantageous due to the fact that the differentwindow coefficients for the different windows do not have to be storedtwo times, with one set for the encoder and one set for the decoder.Instead, due to the fact that in accordance with the present inventionidentical windows and window sequences are used in the encoder and thedecoder, only a single set of window coefficients has to be stored.Hence, the memory usage of the inventive mobile device illustrated inFIG. 1d is substantially reduced with respect to a different concept inwhich the encoder and the decoder have different windows or in whichcertain post-processing with processing other than windowing operationsis performed.

Subsequently, reference is made to a further embodiment with respect tothe transform/transform length switching embodiment.

The transform and overlap length-adaptive coding scheme outlined abovewas implemented in the transform coded excitation (TCX) path of theLD-USAC encoder, a low-delay variant of xHE-AAC [5] with a frame lengthof 20 ms, and tested at 48 kbit/s mono. At this configuration point,LD-USAC operates in TCX-only mode with a core-frame length of 512samples and a long-transform overlap of 256 samples, i.e. 33%, during(pseudo-) stationary input conditions. The encoder includes a transientdetection unit, whose output is input to a transform length determiningunit and to the inventive overlap width determining unit. Threetransform lengths are available for coding: a TCX-20 length with 512MDCT coefficients, a TCX-10 length with 256 MDCT coefficients, and aspecial TCX-5 length with 128 MDCT coefficients. Accordingly, one ofthree overlap widths can be used and transmitted per frame: maximumoverlap of 256 core-samples (10 ms), half overlap of 128 core-samples (5ms), and minimum overlap of 16 samples (0.6 ms). For each frame thetransform lengths have to be selected such that the sum of the lengthsof all transforms in that frame equals the core-frame length, i.e. 512samples.

In an embodiment of the inventive coding system, the encoder operates asfollows:

-   1. The transient detection unit identifies the presence and, if    applicable, the location of the onset of the strongest transient in    the new signal portion of a given frame (i.e. excluding the overlap    region between the current and previous frame). The resolution of    the index describing the transient location is ⅛ of the frame    length, so the index range is 0, . . . , 7.-   2. If no transient has been detected, or if the transient location    index is 6 or 7, the affected frame is coded using the TCX-20    transform by decision of the transform length determining unit.    Otherwise, a combination of TCX-10 and/or TCX-5 transforms is used:    either 2×TCX-10 or 4×TCX-5 or TCX-10 followed by 2×TCX-5 or 2×TCX-5    followed by TCX-10.-   3. The overlap width determining unit now controls the overlap    shapes of the transforms used within the current frame (excluding    the already chosen overlap with the last frame) according to the    objectives enumerated above, such that the longest possible overlaps    which do not violate said objectives are selected. In particular, if    a frame is TCX-20 and the transient location index is 6 or 7, the    overlap unit returns minimum or half overlap, respectively. If no    signal nonstationarity is present in a frame, maximum overlap is    used.-   4. Furthermore, if a TCX-10/-5 combination was returned by the    transform length determining unit for the (non-stationary) frame,    the overlap width determining unit controls the exact composition of    transform lengths in that frame. Particularly, if maximum overlap is    used in the preceding as well as the current frame, 2×TCX-5 followed    by a TCX-10 are applied in the current frame, with the first of the    TCX-5 transforms being the inventive transition transform with    double overlap. If either the last frame's or the current frame's    overlap width is less than maximum, one of the mixed TCX-10/-5    configurations is also used. If both last and current frame have    less than maximum overlap, 4×TCX-5 is used.-   5. The encoder now proceeds to the windowing of the signal and the    actual MDCTs for the frame. Special care has to be taken regarding    the order of the windowing operations in the presence of the    inventive double-overlap transition window in order to attain    perfect reconstruction after decoding. The remainder of the encoding    process is similar to that of xHE-AAC. TNS is optionally applied to    the individual transforms, and grouping of two TCX-5 MDCT    coefficient sets into one TCX-10-like set of (interleaved)    coefficients may be performed to save side information. For each    frame, one overlap width value as well as one 1-bit flag indicating    TCX-20 or non-TCX-20 coding is transmitted to the decoder.

Like the encoder, the appropriate decoder according to the embodimentfeatures an overlap width determining unit interpreting the transmittedoverlap width values to control the length and windowing of the inverseMDCTs so that encoder and decoder are fully synchronized with regard tothe transforms used. As in the encoder, the order of the windowing andfolding operations after the individual MDCTs is critical to obtainperfect signal reconstruction.

Subsequently, a further embodiment of the invention is discussed andillustrated in the context of FIGS. 8 to 15 f. This aspect, which isalso named the “multi-overlap aspect” can be combined with the overlapwidth and transform length switching embodiment discussed with respectto FIGS. 1 to 7 or can be implemented separately from this aspect.

An encoder-side of the invention is illustrated in FIG. 8a and adecoder-side is illustrated in FIG. 8b . In particular, the apparatusfor generating an encoded signal or the encoder illustrated in FIG. 8acomprises a window sequence controller for generating a window sequenceinformation 809 forwarded, for example, to a pre-processor 802, aspectrum converter 804 or an output interface 810 as illustrated in FIG.8a . The window sequence information indicates a first window functionfor generating a first frame of spectral values, a second windowfunction and one or more third window functions for generating a secondframe of spectral values. The first window function, the second windowfunction and the one or more third window functions overlap within amulti-overlap region.

This multi-overlap region is, for example, illustrated at 1300 in FIG.13 or FIG. 14b or FIG. 15e or FIG. 15f . Thus, in this multi-overlapregion 1300, at least three window functions, i.e. the first windowfunction with respect to FIG. 15f illustrated at 1500, the second windowfunction 1502 and the third window function 1503, overlap each otherwithin the multi-overlap region 1300. There can also be a higheroverlap, such as an overlap of four, five or even more windows.Alternatively, FIG. 15e illustrates the situation where one again hasthe first window function 1500, the second window function 1502 but nowfour third window functions 1503 in contrast to a single third windowfunction 1503 of FIG. 15 f.

In order to correctly handle this multi-overlap region which results ina significant reduction of the delay necessitated for the transientlook-ahead region, a pre-processor 102 is provided. The pre-processor isconfigured for windowing a second block of samples corresponding to thesecond window and the one or more third window functions using anauxiliary window function to obtain a second block of windowed samples.Furthermore, the pre-processor is configured for pre-processing thesecond block of window samples using a folding-in operation of a portionof the second block overlapping with the first block into themulti-overlap portion to obtain a pre-processed second block of windowedsamples having a modified multi-overlap portion. Furthermore, a spectrumconverter 804 is configured for applying an aliasing-introducingtransform to the first block of samples using the first window to obtainthe first frame of spectral values. Furthermore, the spectrum converteris configured for applying an aliasing introducing transform to a firstportion of the pre-processed second block of windowed samples using thesecond window function to obtain a first portion of spectral samples ofa second frame and for applying the aliasing introducing transform to asecond portion of the pre-processed second block of windowed samplesusing the one or more third window functions to obtain a second portionof spectral samples of the second frame. Furthermore, a processor 806indicated as “encoding processor” is provided within the encoder of FIG.8a for processing the first frame and the second frame of spectralvalues to obtain encoded frames of the audio signal at the output 807 ofblock 806. Thus, the encoding processor can be identical or differentfrom the encoding processor 110 of FIG. 1a and can perform any of thewell-known MPEG or AMR r any other encoding features in the art.

Subsequently, reference is made to FIG. 13. FIG. 13 once againillustrates the second half of the first window function 1500, thesecond window function 1502 and, in the second picture of FIG. 13, twothird window functions 1503. Contrary thereto, the upper illustration inFIG. 13 illustrates again a first window function 1500, a second windowfunction 1502 and, in contrast to for example FIG. 15f and slightlysimilar to FIG. 15e , four third window functions 1503. Alternatively,the number of third window functions can also be three, five or so.

Furthermore, FIG. 13 additionally illustrates a situation with adifferent first window function 1500′, a different second windowfunction 1502′ and the same third window function 1503. The differencebetween 1500 and 1500′ is that the overlap length of the functions 1500′and 1502′ is half with respect to windows 1500, 1502. Thus, thesituation of window functions 1500′ and 1502′ is that the overlap lengthis a half overlap illustrated at 218, for example in FIG. 2d , while thefull overlap length corresponds to a complete frame as, for example,illustrated at 203 in FIG. 2a or FIG. 13. Hence, the window functions1500′ and 1502′ illustrated in this figure represent a combination ofthe multi-overlap aspect and the overlap width determination aspect.

In order to better explain the procedure of the preprocessor 802 on theencoder-side, reference is made to the illustration in FIG. 11a on theone hand and flowcharts in FIG. 9a, 9b on the other hand. Regarding thedecoder, reference is made to the corresponding illustrations in FIG. 8b, FIGS. 10a, 10b and the illustration in FIG. 11b . Furthermore, theencoder is also illustrated in FIG. 12a and the decoder is illustratedin FIG. 12 b.

In particular, FIG. 11a illustrates once again the first window function1500 and at least a portion of the second window function 1502 andeither four third window functions 1503 or a single third windowfunction 1503. In particular, FIG. 11a additionally illustrates theauxiliary window function 1100. The auxiliary window function 1100 has afirst portion 1100 a coinciding with the first ascending portion 1500 aof the first window function 1500. Furthermore, the auxiliary windowfunction 1100 has a second non-overlapping part 1100 b addvantageouslyhaving window coefficients equal to unity and a third portion 1100 ccorresponding to a descending or falling or right portion of the one ormore third window functions. Thus, the auxiliary window function 1100covers the second half of the earlier frame illustrated at 1102, thefirst half of the current frame i indicated by 1103, the second half ofthe current frame i indicated by 1104 and the first small portion 1105covered by the auxiliary window function part 1100 c. As becomes clearfrom FIG. 11a , the auxiliary window function is treated as a “startwindow sequence” or corresponds to such a “start window sequence”, as ifin the frame i+1 a sequence of short windows would have to beintroduced. Importantly, however, a sequence of short windows is alreadyintroduced in the current frame rather than in the coming frame i+1.

The functionality of the preprocessor is then illustrated in FIG. 11a .The preprocessor preprocesses the second block of window samplesobtained by windowing using the auxiliary window function using a votingin operation indicated as “start fold-in alias, frame i”. Thus, the leftmost portion of the second block of windowed samples indicated by 1110is folded inward. This portion 1110 is the portion of the second blockof windowed samples overlapping with the preceding first window function1500, i.e. the portion of the second block of windowed samplescorresponding to the time period 1102 and lying in the preceding framei−1. Due to the fact that this folding-in operation of the portion 1110now influences the overlap region 1300, the folding-in operationperformed by the pre-processor results in a modified multi-overlapportion. Now, the spectrum converter applies the operation illustratedin the line of FIG. 11a indicated as “inner fold-in aliases”. Inparticular, the spectrum converter applies an aliasing introducingtransform to the first block of samples using the first window functionillustrated for frame i−1. The aliasing-introducing transform comprisesthe fold-in operation illustrated at 1120 and the subsequent, forexample, DCT-IV transform indicated at 1122. To this end, the firstwindow function 1500 is necessitated in order to obtain the shape beforethe folding-in operation 1120 for frame i−1. Furthermore, the spectrumconverter applies the aliasing-introducing transform to the firstportion indicated by item 1131 in FIG. 11a . This is done using thesecond window function 1502 and in particular the right portion of thesecond window function 1502. This operation results in a first portionof spectral samples of a second frame obtained by transform 1132, wheretransform 1132 once again represents a DCT-IV operation whichconstitutes, together with the corresponding folding operation, but nowonly in the right overlapping portion of block 1131, thealiasing-introducing transform.

Furthermore, the spectrum converter is configured to apply the aliasingintroducing transform to a second portion 1133 of the pre-processedsecond block 1130 using the one or more third window functions 1503 toobtain a second portion 1135 of spectral samples of the second frame.Thus, in order to obtain the second portion 1135 of spectral samples,four N/8 DCT-IV transforms or a single N/2 DCT-IV transform can beapplied. The number of transforms and the lengths depend on the numberof third window functions. Generally, the length, the transform or thenumber of spectral samples in the second portion 1135 is equal to thenumber of spectral samples in a frame minus the length of the transform1132 and the result is then divided by the number of third windowfunctions used.

Thus, the pre-processor 802 is generally operative for windowing 902(FIG. 9a ), the audio signal using the auxiliary window function 1100 toobtain the second block of windowed samples. Then, the processor 904advantageously applies the folding operation indicated at 1110 in FIG.11a to obtain the pre-processed second block of windowed samples withthe modified multi-overlap portion 1300. Then, the converter 906 appliesthe transforms using the first, the second and the third windowfunctions to obtain the first frame of spectral values 1122, the firstportion 1132 of the second frame and the second portion 1135 of thesecond frame or frame i in the notation of FIG. 11 a.

In the embodiment, illustrated with respect to FIG. 9b , the auxiliarywindow function is determined 910 by making reference to the firstwindow function, and exemplarily by selecting, as the first portion 1100a of the auxiliary window function 1100, the first portion 1500 a of thefirst window function. Furthermore, the non-overlapping part 1100 b isdetermined (window coefficients of one are taken for the correspondinglength) and the third part 1100 c is then determined, again exemplarilyby taking the second part of the short window function.

Then, the audio signal is windowed 912 with this auxiliary windowfunction in the correct relation to the preceding or first frame i−1illustrated in FIG. 11a . Then, as illustrated at 914 in FIG. 9b , theleft portion 1110 and advantageously the right portion 1111 arefolded-in. In step 916, a folding of the overlapping portionsillustrated in hatched lines in item e) or f) in the inner region isperformed. Furthermore, as illustrated at 918, if there are more thirdwindow functions as in FIG. 11a sub-picture e), then the fold-in ofoverlapping portions of the third window functions is performed as well.However, if there is only a single third window function as illustratedin FIG. 11a , sub-picture f), then the control proceeds from step 916 to920 directly without step 918. In step 920, DCT operations are performedusing the shorter DCT kernels than the DCT kernel for the first frame.The DCT kernel for sub-picture e) is, for the second window function,N/2, and for the third window functions, N/8. Contrary thereto, whenthere is only a single third window function, then the transform kernelis equal to N/2 for the second window function and is equal to N/2 forthe single third window function.

Thus, it becomes clear that the multi-overlap region 1300 is windowedtwo times. The first windowing is done by the first portion 1100 a ofthe auxiliary window and the second windowing is performed by the secondhalf of the first third window function 1503 as illustrated insub-picture e) or f) of FIG. 11 a.

Reference is made to FIG. 13 again. As discussed in the context of FIG.1a or in the context of FIG. 8a , the window sequence controllergenerates the specific window shapes. In an embodiment, the windowsequence controller is configured to comprise the transient locationdetector 106. When a transient is detected in transient detectionportions 0 or 1, then the encoder is controlled to go into themulti-overlap portion mode so that these transients indicated at 1305are confined to lie only within a single third window or within twoadjacent third windows. Specifically, the left transient 1305 isconfined to lie in the first short window function only, where the righttransient of the transients 1305 lies in the first to third windowfunctions. However, when it is determined that the transients arelocated in a region different from 0, such as in region 1, 2, 3 or so,then a processing without the multi-overlap region can be performed, forexample, similarly as discussed in the context of FIG. 6a , FIG. 6b ,FIG. 7 or so.

Contrary thereto, however, the multi-overlap region processing can alsobe performed in the context of the window switching application, where,when a transient is detected, an even larger set of short windows can beswitched for the current frame so that, advantageously within one andthe same block or frame raster, either a long window or a specifiednumber of short windows is used for windowing. The first windowcorresponds to window 1500, for example in FIG. 13, the second windowcorresponds to window 1502 and a switch-over is performed, withouthaving reference to a certain transient location, to a number of thirdwindow functions only when anywhere in the current frame a transient isdetected without knowing where exactly within the frame the transient islocated.

However, it is advantageous in order to keep the number of third windowfunctions as small as possible that the switching into the multi-overlapportion mode and the additional switching of the transform overlap andthe transform length selection is performed depending on the specificlocation of the transient within the frame, i.e. in one of four or eveneight different portions of a frame or of a time portion correspondingto a frame, where this time portion is then equal to half the size of along window, such as long window 1500 of FIG. 13. Advantageously, themulti-overlap portion is, as can be seen in FIG. 13, located before astart 208 (illustrated in FIG. 2 on the one hand and FIG. 13 on theother hand) of the look-ahead region.

On the decoder-side, an analogous processing is performed. In anembodiment of an apparatus for decoding an encoded audio signal 821,which comprises an encoded first frame and an encoded second frame, adecoding processor 824 of FIG. 8b is necessitated for processing thefirst encoded frame and the second encoded frame to obtain a first frameof spectral values and a second frame of spectral values, the first andthe second frames comprising aliasing portions. A time converter 826 isconnected to the decoded processor 824 and the time converter 826 isconfigured for applying a transform to this first frame using a firstwindow function to obtain a first block of samples. Furthermore, thetime converter 826 is configured to applying the transform to a firstportion of the second frame using a second window function and to applythe transform to a second portion of the second frame using one or morethird window functions to obtain the second block of samples. Asdiscussed in the context of FIG. 1a , the first window function 1500,the second window function 1502 and the one or more third windowfunctions 1503 together have a multi-overlap region 1300.

Furthermore, the decoder comprises a post-processor 828 forpost-processing the second block of samples using a folding-outoperation to obtain a post-processed second block of samples having aportion of the second block of samples overlapping with the first blockof samples in the multi-overlap region. Furthermore, the post-processor828 is configured for windowing the post-processed second block ofsamples using the auxiliary window function discussed in the context ofFIG. 8a and FIG. 11a . The post-processor 828 performs an overlap-addingof the windowed post-processed second block of samples and the firstblock of samples to obtain the decoded audio signal indicated at 829 ofFIG. 8b or at block 175 of FIG. 1c . Thus, basically the post-processor828 of FIG. 8b may have the functionality of the synthesis windower 172with respect to the auxiliary window function and the overlap-adder 174.

Subsequently, the functionality of the post-processor in cooperationwith the time converter is discussed with respect to the illustration ofFIG. 11b which illustrates a reverse processing with respect to the FIG.11a encoder illustration. The first frame of spectral values 1142 isinput into an N-size inverse transform 1161 and the first portion 1152of the second frame is input into an N/2 inverse transform 1162 anddepending on the number of third window functions, the second portion1155 of the second frame is either input into four N/8 short transforms1163 or a single N/2 transform 1162 similar as for the first portion1152 of the second frame.

This procedure is performed by the time converter. The time converteradditionally uses the first window function to perform the windowingtogether with a before performed folding-out operation illustrated at1170 in FIG. 11b . Furthermore, the second window function is used whenapplying the procedures to the first portion 1152 illustrated at 1172.Specifically, the fold-out of specifically the most right portion 1173of the second window function and the second subsequent windowing isperformed while, on the left hand side of the frame, any inner-fold-outis not performed. Furthermore, the transform performs a specificfold-out and subsequent windowing and additional overlap-adding not onlywith the first portion 1152 of the second frame but also with the secondportion 1155 of the second frame as illustrated at 1172 in FIG. 11b . Ifthere is only a single third window function illustrated at sub-picturef) in FIG. 11b , then only a single folding-out operation from bothsides together with the windowing using the right hand portion of thesecond window function and the left hand portion of the third windowfunction and the subsequent overlap-add within the overlapping range1174 are performed.

Then, the post-processor applies the post-processing using thefolding-out operation illustrated at 1175 with the first portion of theresult of the procedure in 1172 to obtain a portion 1176 a extending inthe previous frame and advantageously 1176 b extending in the nextframe. Then, windowing with the folded-out portion 1176 a, 1176 b and ofcourse with the portion within the current frame i using the auxiliarywindow function is performed to obtain the state illustrated at 1175.Then, a final overlap-adding of the auxiliary window function-windowedpost-processed second block of samples and the first block of samples isperformed at and within the overlapping range 1180 to obtain the finaldecoded audio signal corresponding to this overlapping range 1180.Furthermore, this procedure additionally results in a subsequent portionof decoded audio signal samples 1181 due to the fact that there is nooverlap and the next section 1182 is obtained by overlapping with thecorresponding part of a window function for frame i+1, following frame iin time.

Thus, as illustrated in FIG. 10a , the decoder-side method comprisesapplying 1000 a transform to the first frame using the first windowfunction and applying 1010 the transform to the first portion of thesecond frame using the second window function and applying 1020 thetransform to the second portion of the second frame using the thirdwindow function(s). Then, in step 1030 a folding-out operation isperformed and in step 1040, a windowing using the auxiliary windowfunction is performed and finally, in step 1050 an overlap-adding of thewindowed post-processed second block and the first block is performed toobtain the decoded audio signal at the end of the processingillustrated, for example, in FIG. 11 b.

As illustrated in FIG. 10b , embodiments comprise performing an inverseDCT operation for each portion of the second frame, i.e. performingseveral DCT operations with shorter lengths with respect to the previousframe i−1, where a long window 1500 was used. In step 1070 a folding-outof the inner aliasing portions is performed as the operation illustratedin 1172 and the folding-out is advantageously a mirroring at thecorresponding border illustrated as vertical lines in the line indicatedby 1172 in FIG. 11b . Then, in step 1080, a windowing using the secondand third window functions within the block 1184 is performed and thesubsequent overlap-adding of the windowing result within the block isperformed as illustrated in 1090. Then, as indicated at 192, afolding-out of the left/right or, in other words, earlier/later aliasingportions of the overlap-add result is performed in order to obtainportions 1176 a extending in the previous frame and portion 1176 bextending in the next frame. However, the representation in 1175 is onlysubsequent to windowing using the auxiliary window function illustratedin 1094. Then, in step 1906, an overlap-adding with the first block ofsamples is performed subsequent to the windowing using the auxiliarywindow function.

Subsequently, reference is made to FIG. 12a and FIG. 12b . Item a inFIG. 12a corresponds to the procedure in the first line of FIG. 11a .The procedure in sub-picture b) corresponds to the procedure performedin the second and third lines of FIG. 11a and the procedures illustratedin item c) in FIG. 12a correspond to the procedures in the last twolines of FIG. 11a . Analogously, the decoder-side representationcorresponds to FIG. 12b . In particular, the first two lines of FIG. 11bcorrespond to the sub-picture f) in FIG. 12b . The third and fourthlines correspond to item e) in FIG. 12b , and the last line in FIG. 12bcorresponds to the last line in FIG. 11 b.

FIG. 14a illustrates a situation where the window sequence controller onthe encoder-side or the elements 824, 826, 828 of the decoder side areconfigured for switching between a non-multi-overlap situation as inFIG. 14a and a multi-overlap situation illustrated in FIG. 14b . Thus,when a transient is detected in transient portion 0, one procedure is tonot apply the multi-overlap portion but to switch to single-overlappingshort windows TCX-10 from TCX-20 windows. Advantageously, however, aswitch over to a multi-overlap portion is performed by applying a windowsequence comprising the first window 1400, the second window 1402 andone or, in the embodiment of FIG. 14b , two third windows 1403.

The window overlaps and sizes of FIG. 14b are somewhat different fromthe illustration in FIG. 13, but it becomes clear that the generalprocedures regarding the encoder-side in FIG. 11a or the decoder-side inFIG. 11b take place in the same way.

Subsequently, FIG. 15 is discussed. Specifically, FIG. 15 illustrates,as the black boxes, a transient detection look-ahead 1590 and theduration of the resulting pre-echo 1595. FIG. 15a illustrates atraditional High-Efficiency-AAC-type sequence comprising a long-startwindow, eight short windows, a long-stop window and so on. Thenecessitated look-ahead is high and amounts to N+N/2+N/16, but thepre-echo 1595 is small. Analogously, FIG. 15b illustrates a traditionalAAC low delay-type transient detection procedure resulting in a windowsequence comprising a long sequence, a long-start window, a low overlapwindow, and a long-stop window. The transient detection look-ahead isthe same as in FIG. 15a , but the duration of the pre-echo is longerthan in FIG. 15a . On the other hand, however, the efficiency is higherdue to the fact that the more shorter windows are used, the lower thebitrate efficiency is.

FIGS. 15c and 15d illustrate an implementation of the High-EfficiencyAAC or an AAC-low delay procedure with a reduced transient detectionlook-ahead of N/16 samples and there are only shown long sequencespossible with a reduced transient detection look-ahead of N/16 samples.If the sequence consists of a long window, a long window, a long-startwindow, a long-stop window, and so on, as illustrated in FIG. 15d , onlythe post-echo is reduced compared to FIG. 15c , but the pre-echo 1595 isthe same. Hence, FIG. 15c, d , illustrate a short look-ahead similar tothe inventive FIGS. 15e and 15f . If one would now implement the multioverlap portion as in FIGS. 15c and 15e , then one can only usesequences as in those figures, but any switch to a short window is notpossible. Thus, the multi-overlap portion allows either switching toshort windows to reduce the pre/post echoes, or to use a shortlook-ahead delay or both features to reduce the delay and to reducepre/post echoes.

FIG. 15e illustrates a High-Efficiency AAC sequence with a reducedtransient detection look-ahead of N/16 samples and the multi-overlapregion 1300. The sequence comprises a long window, a further long window1500, a further start sequence 1502, four short sequences 1503, and along-stop window 1504. As becomes clear, the look-ahead is small, as isthe pre-echo. A similar situation is obtained for FIG. 15f illustratinga similar configuration as in FIG. 15e , but with only a single thirdwindow function instead of four short sequences.

Although the present invention has been described in the context ofblock diagrams where the blocks represent actual or logical hardwarecomponents, the present invention can also be implemented by acomputer-implemented method. In the latter case, the blocks representcorresponding method steps where these steps stand for thefunctionalities performed by corresponding logical or physical hardwareblocks.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

The inventive transmitted or encoded signal can be stored on a digitalstorage medium or can be transmitted on a transmission medium such as awireless transmission medium or a wired transmission medium such as theInternet.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disc, a DVD, a Blu-Ray, a CD, a ROM, a PROM, and EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may, for example, be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive method is, therefore, a datacarrier (or a non-transitory storage medium such as a digital storagemedium, or a computer-readable medium) comprising, recorded thereon, thecomputer program for performing one of the methods described herein. Thedata carrier, the digital storage medium or the recorded medium aretypically tangible and/or non-transitory.

A further embodiment of the invention method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may, for example, be configured to be transferredvia a data communication connection, for example, via the internet.

A further embodiment comprises a processing means, for example, acomputer or a programmable logic device, configured to, or adapted to,perform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example, a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are performed by any hardware apparatus.

While this invention has been described in terms of several advantageousembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

REFERENCES

-   [1] International Organization for Standardization, ISO/IEC 14496-3    2009, “Information Technology—Coding of audio-visual objects—Part 3    Audio,” Geneva, Switzerland, August 20096.-   [2] Internet Engineering Task Force (IETF), RFC 6716, “Definition of    the Opus Audio Codec,” Proposed Standard, September 2012. Available    online at http://tools.ietf.org/html/rfc6716.-   [3] C. R. Helmrich, “On the Use of Sums of Sines in the Signal    Windows,” in Proc. of the 13^(th) Int. Conference on Digital Audio    Effects (DAFx-10), Graz, Austria, September 2010.-   [4] J. Herre and J. D. Johnston, “Enhancing the Performance of    Perceptual Audio Coders by Using Temporal Noise Shaping (TNS),” in    Proc. 101^(st) , AES Convention, LA, USA, November 1996-   [5] M. Neuendorf et al., “MPEG Unified Speech and Audio Coding—The    ISO/MPEG Standard for High-Efficiency Audio Coding of All Content    Types,” in Proc 132^(nd) Convention of the AES, Budapest, Hungary,    April 2012. Also to appear in the Journal of the AES, 2013.

1. Apparatus for encoding an audio or image signal, comprising: acontrollable windower for windowing the audio or image signal to providea sequence of blocks of windowed samples; a converter for converting thesequence of blocks of windowed samples into a spectral representationcomprising a sequence of frames of spectral values; a transient locationdetector for identifying a location of a transient within a transientlook-ahead region of a frame; and a controller for controlling thecontrollable windower to apply a specific window having a specifiedoverlap length to the audio or image signal in response to an identifiedlocation of the transient, wherein the controller is configured toselect the specific window from a group of at least three windowscomprising a first window having a first overlap length, a second windowhaving a second overlap length, and a third window having a thirdoverlap length or having no overlap, wherein the first overlap length isgreater than the second overlap length, and wherein the second overlaplength is greater than the third overlap length or greater than anoverlap of zero, wherein the specific window is selected based on thetransient location such that one of two time-adjacent overlappingwindows comprises first window coefficients at the location of thetransient and the other of the two time-adjacent overlapping windowscomprises second window coefficients at the location of the transient,wherein the second coefficients are at least nine times greater than thefirst coefficients.
 2. Apparatus of claim 1, wherein the controller isconfigured to select the specific window so that the first windowcoefficients are equal to 1 and the second window coefficients are equalto zero.
 3. Apparatus of claim 1, wherein the first overlap length isequal to one quarter or one third or one half of a length of thespecific window, wherein the second overlap length is equal to one halfor one third of the first overlap length, and wherein the third overlaplength is one half or one quarter or one eighth of the second overlaplength or less than or equal to 1.25 milliseconds.
 4. Apparatus of claim1, further comprising an output interface for associating a windowinformation provided by the controller with an encoded representation ofthe spectral representation comprising a sequence of encoded blocks ofspectral values.
 5. Apparatus of claim 1, wherein the converter isconfigured for performing a modified discrete cosine transform or amodified discrete sine transform, wherein a number of spectral values ina block of spectral values is lower than a number of windowed samples ina block of windowed samples.
 6. Apparatus of claim 1, wherein thecontroller is configured to control the windower in such a way that aconstant frame raster is maintained in the sequence of blocks comprisingat least five frames, wherein a frame has a length being equal to alength of the first window or being equal to the length of the firstwindow divided by an integer multiple of two, and wherein the transientlook-ahead region extends from a location in the current frame until alocation in a next frame, the location being in the middle of the frameor displaced from the middle of the frame by less than or equal to 25%of the samples of a frame.
 7. Apparatus of claim 1, wherein thetransient location detector is configured to apply the transientlook-ahead region extending over a full frame and to distinguish atleast four quarters, and wherein the controller is configured to selectthe first window, when no transient is detected, to select the secondwindow, when a transient is detected in the first or fourth quarter andto select the third window, when a transient is detected in the secondor the third quarter.
 8. Apparatus of claim 1, wherein the controller isconfigured to determine the specific window so that an overlap width ischosen in such a way that only one of two time-adjacent overlappingwindows comprises the detected transient.
 9. Apparatus of claim 1,wherein the transient location detector is configured to detect thelocation of the transient within the transient look-ahead region so thatthe transient location coincides with a start or an onset of thetransient or coincides with a time location of a maximum amplitude orenergy or a center of an energy of the transient.
 10. Apparatus of claim1, wherein the transient location detector is configured for using atransient look-ahead region covering a second portion of a current frameand a first portion of a next frame, wherein the transient locationdetector is configured to identify eight different transient locationsidentified by an index from zero to seven, wherein the controller isconfigured to select the second window, when the transient index isseven or to select the third window, when the transient index is six.11. Apparatus of claim 1, wherein the transient location detector isconfigured for using a transient look-ahead region covering a secondportion of a current frame and a first portion of a next frame, whereinthe transient location detector is configured to identify eightdifferent transient locations identified by an index from zero to seven,wherein the controller is configured to select the first window, when notransient is detected, to select the second window, when the transientindex is equal to zero, one, six, seven, or to select the third window,when the transient index is equal to two, three, four, five. 12.Apparatus of claim 1, wherein the controller is configured to select anumber of specific windows for a frame so that a number of spectralvalues acquired by a plurality of windows for a frame is equal to thenumber of spectral values acquired by converting a window functionhaving the first window size, wherein the first to third windows have anidentical first window size, and wherein a frame is defined by theidentical window size, and wherein the group comprises a fourth windowand a fifth window, the fourth window and the fifth window having anidentical second window size, the second window size being an integerfraction of the first window size, and wherein the fourth window has afourth overlap length and the fifth window has a fifth overlap lengthlower than the fourth overlap length.
 13. Apparatus of claim 1, whereinthe controller is configured to determine, based on the transientlocation, an overlap width information and a short/long transforminformation, wherein the overlap width information is determined to havea first value indicating a maximum overlap width, a second valueindicating a medium overlap width and a third value indicating a minimumoverlap width, wherein the short/long transform information comprises anidentification for a long window or a short window for a framecomprising more than one transforms, and wherein the controller isconfigured to determine a single window for a frame or a sequence oftwo, three or four windows for the frame based on the overlap widthinformation and the short/long transform information.
 14. Apparatus ofclaim 1, wherein the controller is configured to determine a number oftransforms and corresponding lengths of the transforms for a frame inresponse to an identified transient location, wherein the controller isconfigured to determine the number and lengths of the transform for aframe such that a number of spectral values acquired by the transformsfor the frame is equal to a number of spectral values acquired by alongest transform length associated with a maximum overlap windowfunction.
 15. Apparatus of claim 1, wherein the controller is configuredto determine a length and number of transforms or to select a specificwindow so that a multi-overlap region extending over a plurality ofsamples is acquired within the frame so that the plurality of samplesare windowed by at least three overlapping windows.
 16. Apparatus ofclaim 1, wherein the controller is configured to determine the lengthand number of transforms or to select the specific window so that themulti-overlap region is determined, wherein a transient is located at alocation of the transient look-ahead region corresponding to a regionextending by 30% of the samples of the frame around the middle of theframe.
 17. Decoder for decoding an audio or image signal comprising asequence of blocks of converted windowed samples and associated windowinformation identifying a specific window function for a block out of atleast three different window functions, comprising: a processor forproviding a sequence of blocks of spectral values; a controllableconverter for converting the sequence of blocks of spectral values intoa time domain representation using an overlap-add processing, whereinthe controllable converter is controlled by the window information toapply window functions indicated by the window information to thecorresponding block to calculate a decoded audio or image signal,wherein the window is selected from a group of at least three windowscomprising a first window having a first overlap length, a second windowhaving a second overlap length, and a third window having a thirdoverlap length or having no overlap, wherein the first overlap length isgreater than the second overlap length, and wherein the second overlaplength is greater than the third overlap length or greater than anoverlap of zero.
 18. Decoder of claim 17, wherein the controllableconverter comprises: a frequency-time converter for converting theblocks of spectral values into a time representation; a synthesiswindower for applying a synthesis window function to the timerepresentation of the block of spectral values; and an overlap-adder foroverlapping and adding time-adjacent window time representations toacquire the decoded audio or image signal, wherein the synthesiswindower, the overlap-adder or the frequency-time converter arecontrolled by the associated window information to apply the synthesiswindow and an overlap as indicated by the window information. 19.Decoder of claim 18, wherein the window information comprises atransform length information and an overlap information, wherein acontroller is configured to determine an overlap and a window for ablock of spectral values based on a previous window information and acurrent window information, wherein a current window information eitherindicates a first transform having a first transform length or twosecond transforms each having a second transform length, or a secondtransform having the second transform length and two third transformseach having a third transform length or four transforms having the thirdtransform length, wherein the first transform length is double the sizeof the second transform length, and wherein the second transform lengthis double the size of the third transform length.
 20. Decoder of claim17, wherein the window information comprises a previous overlapinformation indicating an overlap of a later portion of a previouswindow function, and wherein the window information comprises a currentoverlap information indicating an overlap of a later portion, whereinthe controller is configured to determine an earlier portion of a windowimmediately following the previous window in accordance with the overlapinformation of the later portion of the previous window.
 21. Decoder ofclaim 17, wherein the window information for a current frame indicatinga transform length information necessitating a second transform lengthbeing smaller than a first transform length defining the frame, whereinthe controllable converter is configured to apply a sequence of two orfour windows associated with a third transform length, wherein anoverlap between two time-adjacent windows associated with the thirdtransform length is the third overlap length, and wherein the thirdtransform length is lower than the second transform length.
 22. Decoderof claim 17, wherein the first window, the second window and the thirdwindow have associated the same transform size defining a frame. whereinthe first overlap length is one quarter or one third or one half of alength of the first window, wherein the second overlap length is onehalf or one third of the first overlap length, and wherein the thirdoverlap length is one half or one quarter or one eighth of the secondoverlap length or less than or equal to 1.25 milliseconds.
 23. Decoderof claim 22, wherein the controllable converter is configured to apply,in response to the window information, a fourth window having the firstoverlap length, wherein the fourth window has associated a transformsize being half of the transform size of the first to third windows, orwherein the controllable converter is configured to apply, in responseto the window information, a fifth window having the second overlaplength and a sixth window having the third overlap length, wherein thefifth window and the sixth window have associated the same transformsize being a quarter of the transform size of the first to thirdwindows.
 24. Decoder of claim 23, wherein the controllable converter isconfigured to apply, in response to the window information, either asequence of a single fourth window and two sixth windows, wherein thetwo sixth windows have the third overlap length in overlapping windowportions.
 25. Decoder of claim 23, wherein the controllable converter isconfigured to apply, in response to the window information, a sequenceof windows comprising, in the order: a first window, a fourth window, asixth window and a fifth window, a first window, a fourth window and twosixth windows, a second window, a fifth window, two sixth windows and afifth window, a third window and four sixth windows; a third window, twosixth windows, and a fourth window, a third window, three sixth windows,and a fifth window; a first window and a second window, or vice versa, afirst window, a third window or vice versa, or a second window and athird window, or vice versa.
 26. Decoder of claim 17, wherein the windowindication is associated with each frame of the encoded audio or imagesignal, wherein a frame is defined by a transform associated with thefirst window, wherein the window indication is a variable length codecomprising a bit for a transform length and a single or 2 bits for anoverlap length, wherein the controllable converter is configured toapply a sequence of windows defined by an overlap length of a previouswindow indication and by an overlap length and a transform length of acurrent window indication immediately following the previous windowindication in the encoded audio or image signal.
 27. Decoder of claim17, wherein the controllable converter is configured to perform aninverse modified discrete cosine transform or an inverse modifieddiscrete sine transform or any other aliasing reducing transform with anoverlap and add functionality.
 28. Decoder of claim 17, wherein thefirst overlap length comprises 256 samples or about 10 ms, wherein thesecond overlap length comprises 128 samples or about 5 ms and whereinthe third overlap length comprises 16 samples or about 0.6 ms, andwherein the frame length comprises 512 samples or about 20 ms. 29.Decoder of claim 17, wherein a first transform length comprises 512 MDCTor MDST coefficients, wherein a second transform length comprises 256MDCT or MDST coefficients, and wherein a third transform lengthcomprises 128 MDCT or MDST coefficients.
 30. Decoder of claim 17,further comprising: an apparatus for encoding an audio or image signalof claim
 1. 31. Decoder of claim 30, wherein windows used in theapparatus for encoding are identical to corresponding windows used inthe decoder, and wherein the decoder comprises a read-only memory, inwhich only a single set of windows is stored for usage in the apparatusfor encoding and the decoder.
 32. Method for encoding an audio or imagesignal, comprising: windowing the audio or image signal to provide asequence of blocks of windowed samples; converting the sequence ofblocks of windowed samples into a spectral representation comprising asequence of frames of spectral values; identifying a location of atransient within a transient look-ahead region of a frame; andcontrolling the windowing to apply a specific window having a specifiedoverlap length to the audio or image signal in response to an identifiedlocation of the transient, wherein the specific window is selected froma group of at least three windows comprising a first window having afirst overlap length, a second window having a second overlap length,and a third window having a third overlap length or having no overlap,wherein the first overlap length is greater than the second overlaplength, and wherein the second overlap length is greater than the thirdoverlap length or greater than an overlap of zero, wherein the specificwindow is selected based on the transient location such that one of twotime-adjacent overlapping windows comprises first window coefficients atthe location of the transient and the other of the two time-adjacentoverlapping windows comprises second window coefficients at the locationof the transient, wherein the second coefficients are at least ninetimes greater than the first coefficients.
 33. Method for decoding anaudio or image signal comprising a sequence of blocks of convertedwindowed samples and associated window information identifying aspecific window function for a block out of at least three differentwindow functions, comprising: providing a sequence of blocks of spectralvalues; converting the sequence of blocks of spectral values into a timedomain representation using an overlap-add processing, wherein theconverting is controlled by the window information to apply windowfunctions indicated by the window information to the corresponding blockto calculate a decoded audio or image signal, wherein the window isselected from a group of at least three windows comprising a firstwindow having a first overlap length, a second window having a secondoverlap length, and a third window having a third overlap length orhaving no overlap, wherein the first overlap length is greater than thesecond overlap length, and wherein the second overlap length is greaterthan the third overlap length or greater than an overlap of zero, 34.Computer program for performing, when running on a computer or aprocessor, the method of claim
 32. 35. Computer program for performing,when running on a computer or a processor, the method of claim 33.